Toxic effects of mercury persists for generations

PULLMAN, Wash. – Zebrafish exposed to very low levels of methylmercury as embryos not only passed on toxic effects of the chemical exposure to their offspring, but also to the third generation, according to a study that investigated both epigenetic changes – chemical modifications to the DNA – and abnormal neuro-behavior associated with exposure.

The study, by researchers at Washington State University and the University of Wisconsin-Milwaukee, is the first to show that health problems associated with exposure to the neurotoxin are sustained for multiple generations.

“Effects previously observed and suspected now have been shown to be passed to future generations, not simply the individual exposed,” said Michael Skinner, coauthor and founding director of the Center for Reproductive Biology in WSU’s School of Biological Sciences. “This dramatically impacts the health hazards of mercury exposure.”

If the same effects occur in people, it means the health hazards from exposure to mercury, which is present in waterways and fish, is dramatically underestimated, said Michael Carvan, a professor of freshwater sciences at the UW-Milwaukee.

Scientists have already found evidence that methylmercury is a neurotoxin for both children and adults. But those studies have come from examining single, direct exposures to high doses.

The researchers looked at a single cell type – sperm – to compare across three generations, and also tracked two observed abnormal behaviors. The first generation was exposed to a level of methylmercury that could be present in humans who eat a large amount of fish.

Two abnormal behaviors observed across generations after at this level of initial exposure were impaired vision and hyperactivity.

“We tested genome-wide, epigenetic effects and found many genes with alterations,” Carvan said. “We also looked at the two behaviors and each were caused by independently inherited epigenetic effects.”

The effects in the originally exposed generation are not as pronounced in the second and third generations, but more individual fish were affected in the offspring, the researchers found.

Mercury is released into the air from a variety of sources, from power plants to volcanoes, and is washed into bodies of water by rainfall. Once in water, microbes change it into methylmercury, which accumulates in the bodies of fish.

The main way people are exposed to methylmercury is by consuming large amounts of fish.

If the study results translate to people, those who would be most affected are East Asians, Pacific Islanders and Native Americans, populations that eat more fish than most. Among women of childbearing age in those populations, 27 percent have enough of the toxin in their bodies to inflict damage to the nervous system of a fetus.

Carvan said the takeaway message from the study is that women who are of childbearing age should be especially selective in how much and what kind of seafood they eat, especially in Wisconsin, northern Minnesota and the Upper Peninsula of Michigan where fishing in small inland lakes is prevalent. The heavy metal collects at a higher percentage in small lakes.

The extent of exposure also depends on what types of fish are eaten. Store-bought oysters, Pacific salmon and rainbow trout contain much less methylmercury than catfish, swordfish and large, wild-caught fish from small lakes.